Many polymer solutions exhibit a cloud point phenomenon, which occurs at the cloud point temperature, above or below which the polymer is soluble and the solution is clear, but below or above which it becomes insoluble and precipitates and solutions become opaque. In most polymer-solvent systems solubility decreases with falling temperature, but in some cases involving polar polymers, the opposite occurs and the polymer suddenly precipitates at a specific, higher temperature, the cloud-point temperature, or lower critical solution temperature (LCST). If such clear to opaque transitions due to deswelling occur at a low enough temperature and are reversible, such polymer solutions or gels are useful in a variety of applications such as self-activating sunscreens or temperature dependent drug release systems. Polymer solutions exhibiting such reversible phase transitions occurring at low temperatures have been described in Japanese patent Nos. 85-190444; 85-208336; and 86-66707; they consist of aqueous solutions or gels of poly-isopropylacrylamide and of isopropyl acrylamide/N-methylolacrylamide copolymers and of pyrrolidyl or piperidyl/acrylamide copolymers. Besides these acrylamides, N-iso-, N-n-, N-cyclopropylacrylamide and the corresponding methacrylamides are described in these patents, as well as N,N-diethylacrylamide as the only, disubstituted acrylamide. Only these acrylamides or methacrylamides have a sufficiently hydrophobic component as part of their structure to form homopolymers, which can change from a hydrophilic water binding to a hydrophobic water excluding structure at relatively low temperatures.
In copending patent application, Ser. No. 343,979 hydrogels and linear polymers with strong clear--opaque temperature dependent transitions and low LCST in water are described, which consist of copolymers of N,N-dimethylacrylamide with alkyl- and alkoxy-alkyl acrylates in which the alkyl or alkoxy group contains at least an average of 1.2 carbon atoms in the alkyl or alkoxy moiety. It has now been discovered that copolymers of N,N-dimethyl-acrylamide (DMA), which is very hydrophilic, but whose homopolymer by itself does not form a thermo-reversible aqueous solution, and methoxyethyl acrylate or methacrylate (one carbon atom in the alkoxy group) can also be used to make copolymers which as aqueous solutions or gels exhibit LCSTs and temperature dependent reversible cloud points and swelling changes at low temperatures.